This application claims the benefit of Korean Application No. 2003-48596, filed Jul. 16, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates, in general, to a steering system for a vehicle and, more particularly, to a torque detector for a vehicle steering system, which detects torque on a steering wheel.
2. Description of the Related Art
A power steering system provides an auxiliary steering force to wheels using an additional auxiliary driving device, so that a steering force, which should be applied to a steering wheel by a user while a vehicle drives at low speeds and stops, is reduced, thus facilitating a manipulation of the steering wheel. An Electronic Power Steering (EPS) system provides a relatively large auxiliary steering force while a vehicle drives at low speeds and stops, and provides a relatively small auxiliary force while the vehicle drives at high speeds, thus satisfying reduction of the steering force while the vehicle drives at low speeds and stops and driving stability while the vehicle drives at high speeds.
In the EPS system, a direction and magnitude of the auxiliary steering force are determined according to a rotation direction and rotation angle of the steering wheel. For this purpose, a torque sensor is used. A principle of torque detection of the torque sensor is to use variation of magnetic flux around a torque detection coil due to rotation of the steering wheel. A torsion bar is disposed between a wheel driving shaft and a steering wheel driving shaft and the torsion bar is twisted by the rotation of the steering wheel. The twisting of the torsion bar varies the magnetic flux around the torque detection coil, so that a magnitude of an inductance of the torque detection coil varies with the variation of the magnetic flux, thus varying an amplitude of a voltage induced to the torque detection coil. An increase or a decrease in the amplitude of the induced voltage becomes an index that indicates the rotation direction and rotation angle of the steering wheel.
FIG. 1 is a block diagram of a conventional torque detector disclosed in Japanese Patent Publication Hei 8-68703. In the conventional torque detector of FIG. 1, an Alternating Current (AC) voltage output from a current amplifier 31 and an inverse AC voltage output from an inverse current amplifier 32 are applied to both ends of a coil circuit including a coil L1 and a coil L2, respectively. A difference between a torque detection voltage detected in a bridge circuit including the coil L1, the coil L2, a resistor R1 and a resistor R2, and a reference voltage is amplified, and then a torque detection signal TS is obtained by synchronous detection and sampling.
FIG. 2 is a graph showing phase variation of an AC voltage signal VB and a sampling pulse signal SPa in the conventional torque detector of FIG. 1. As shown in FIG. 2, only if phases of the AC voltage signal VB and the sampling pulse signal SPa coincide with each other, accurate synchronous detection and sampling may be achieved. If the phases of the AC voltage signal VB and the sampling pulse signal SPa do not coincide with each other, sampling errors are generated as shown in FIG. 3. FIG. 3 is a graph showing variation of an output voltage of a sample-and-hold circuit 26 according to phase variation of the AC voltage signal VB in the conventional torque detector of FIG. 1. As shown in FIG. 3, if the phase of the AC voltage signal VB does not coincide with the phase of the sampling pulse signal SPa like VB xe2x80x3 shown in FIG. 2, an inaccurate peak value may be sampled like VSAxe2x80x3 shown in FIG. 3. If an accurate peak value is not sampled in a sampling time ts, a torque detection signal TS output from a voltage-to-current converter 39 is no longer valid.
Accordingly, it is an aspect of the present invention to provide a torque detector, which allows accurate torque detection even though a phase of a voltage applied to a coil circuit including a temperature compensation coil and a torque detection coil is destabilized by disturbance, such as temperature variation.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing and other aspects of the present invention are achieved by providing a torque detector, including a synchronous detector to detect an AC voltage signal having a preset DC voltage level and a certain frequency, and generate a detection output signal, a bridge circuit in which a torque detection coil whose inductance varies with rotation of a steering wheel and a temperature compensation coil whose inductance varies with temperature variation are connected in series to each other, the detection output signal and the DC voltage being applied to both ends of the two connected coils, respectively, the bridge circuit allowing a first detection voltage to be induced at a connecting point between the two coils by variation of the inductance of the two coils, and a signal converter to generate a torque detection signal having an amplitude corresponding to a difference between peak values of a preset reference voltage and the first detection voltage.